November 25, 2013

Genes Reveal How Whales Evolved For Ocean Life

Approximately 54 million years ago, whales and other cetaceans diverged from land-dwelling mammals and gradually evolved to live a productive life at sea.

A new study published in the journal Nature Genetics on Sunday revealed genetic evidence of how whales evolved the ability to dive deep into the ocean for long periods of time and how they developed a specialized feeding system that uses baleen instead of teeth.

In the study, a team of international geneticists sequenced the genomes of three minke whales, a fin whale, a bottlenose dolphin, and a finless porpoise. These genomes were analyzed for adaptations to physiological stresses caused by a lack of oxygen, increased reactive oxygen species, and a high-salt living environment. The analysis showed that the gene families affiliated with stress-responsive proteins and anaerobic metabolism were expanded in these cetaceans during their evolution.

One of the most dramatic adaptations for a whale or porpoise is the ability to dive deep into the ocean, which can induce a condition called hypoxia. Under hypoxic conditions, the mammalian body may generate more reactive oxygen species (ROS), compounds that are particularly damaging to DNA. In their analysis, study researchers found evidence showing that when whales or porpoises are under hypoxic or oxidative stress, their bodies increased the production of glutathione, a well-studied antioxidant that prevents cell damage.

The study researchers also found evidence of the genetic mechanism behind the evolution of baleen, a filter-feeding system found inside the mouths of minke whales and other Mysticeti whale species. While previous studies have reported that the ENAM, MMP, and AMEL genes might play a role the development of whale teeth or baleen, the new study concluded that these are pseudogenes. The international team also found that the genes linked to whale's body hair and sensory receptors contracted over the course of evolution.

"Minke whale is the first marine mammal that has been sequenced with such high-depth genome coverage,” said study author Xuanmin Guang, a scientist from the Beijing Genomics Institute in Shenzen. He explained that he genome data not only can help us know much more about the adaptation mechanisms underlying the minke whale, but also provides an invaluable resource for future studies of marine mammals including disease control and prevention, and species conservation and protection."

The animal resembled a small deer and had a thick outside layer on its bones, much thicker than in other mammals of the same size. This adaptation is often seen in modern semi-aquatic mammals like the hippopotamus. A chemical analysis of the animals’ fossilized teeth showed oxygen isotope ratios that resemble those of aquatic animals.

“This remarkable research demonstrates that the study of the structure and composition of fossil bones can tell us about how the skeleton of whales and, by extension, other mammals like humans, interacts with the environment and changes over time,” said Walt Horton, a vice president of research at NEOUCOM at the time.